LaMotte CON 6 PLUS Manual page 29

In practice, conductivity measurements involve determining the current
through a small portion of solution between two parallel electrode
plates when an AC voltage is applied. Conductivity values are related to
the conductance (and thus the resistance) of a solution by the physical
dimensions—area and length—or the cell constant of the measuring
electrode. If the dimensions of the electrodes are such that the area of
the parallel plates is very large, it is reasonable that more ions can reside
between the plates, and more current can be measured. The physical
distance between the plates is also critical, as it aff ects the strength of the
electric fi eld between the plates. If the plates are close and the electric
fi eld is strong, ions will reach the plates more quickly than if the plates
are far apart and the electric fi eld is weak. By using cells with defi ned
plate areas and separation distances, it is possible to standardize or
specify conductance measurements.
Thus derives the term specifi c conductance or conductivity.
The relationship between conductance and specifi c conductivity is:
Specifi c Conductivity, S.C. =
where C is the conductance (siemens)
k is the cell constant, length/area or cm/cm
Since the basic unit of electrical resistance is the ohm, and conductance
is the reciprocal of resistance, the basic unit of conductance was
originally designated a "mho"—ohm spelled backwards—however,
this term has been replace by the term "siemens". Conductivity
measurements are reported as Siemens/cm, since the value is measured
between opposite faces of a cell of a known cubic confi guration. With
most aqueous solutions, conductivity quantities are most frequently
measured in microsiemens per cm (μS/cm) or millisiemens per cm (mS/
cm).
The salinity value which ranges from 2 to 42 is a measure of all salts, not
just sodium chloride. This scale was originally devised for seawater, and
is based on seawater at 15 °C having a conductivity equivalent to that of
a potassium chloride solution of a known concentration. This solution
(0.44 molar) is defi ned as having a salinity of 35 ppt.
The total dissolved solids scale approximates the ppm TDS in surface
waters by multiplying the conductivity of a sample by a factor, 0.66.
Some users prefer the use of resistivity units to describe their water,
(Conductance) (cell constant, k)
= siemens * cm/cm
= siemens/cm
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